make the jobs back button a link, delete the backbutton nobody used

The audit said adopt the shared BackButton. It is not shared: zero importers
since the initial commit, no barrel entry, and a label-plus-underline shape
that fits none of the icon-only back controls here. Adopting it would have
redesigned the Jobs header under cover of a navigation fix.

So: a Link, matching what ScriptJobDetail and DownloadJobDetail already do,
and the dead component goes. useNavigate had no other caller in
PipelineJobDetail and goes with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/workspaces/components/BackButton.tsx

@@ -1,13 +0,0 @@
-import { Link } from 'react-router';
-import { ArrowLeft } from 'lucide-react';
-import { cn } from 'helpers/cn';
-
-type BackButtonProps = { label: string; href: string; className?: string };
-export const BackButton = ({ label, href, className }: BackButtonProps) => {
-  return (
-    <Link to={href} className={cn('flex items-center text-lg text-gray-600 underline dark:text-gray-300', className)}>
-      <ArrowLeft className="mr-1 h-4 w-4" />
-      {label}
-    </Link>
-  );
-};